“Of all the gun tragedies, this hit the closest to home for a lot of us,” Marshall said. “I felt like I had to do something or else I would really be complicit in a company that was continuing to sell firearms.”

Marshall said he sent the message to an e-commerce email chain that includes about 20,000 people and posted it in a Slack online messaging channel that reaches every Walmart associate in the United States.

“In light of recent events, and in response to Corporate’s inaction, we are organizing a ‘sick out’ general strike to protest Walmart’s profit from the sale of guns,” said the letter, which Marshall also provided to CNN.

“Walmart is a company that has always placed its associates and customers first, and we have recently made great strides toward fostering a safe, inclusive, and progressive community. Last year, Walmart raised the minimum age to buy a firearm or ammunition from 18 to 21 and removed products resembling assault-style rifles from its inventory. Walmart is still, however, the single largest retailer of firearms in the United States.

“We have made great strides already, but now we must organize to shape this company into a place we can all be proud of. As associates, we have the power, ability, and opportunity to change this company for the better.”

A coworker sent a second letter urging workers who could not call out sick to wear black in solidarity and to sign a petition on Change.org calling for the chain to stop selling guns.